Boilers

An easy one to start with. Normally, a boiler will keep a bit of water pre-heated all the time so your taps run hot more quickly. It's nice to get instant hot water, but possibly not nice enough to justify firing up your boiler when it's not needed.

Luckily, the eco button on my Worcester Bosch combi boiler disables this function, so the water heater only kicks in when you turn on the hot tap. The delay will vary a bit from house to house, but in my flat it takes about 10 seconds for water to run hot.

Cars

Cars are a prime example of how words like 'eco' mean very different things to different people. Every company has its own interpretation, but pressing your car's eco button will generally do some combination of the following things:

Damp down the throttle response, so you accelerate more smoothly. Note that this only applies up to about half power. If you floor the accelerator, the car assumes you've got a good reason (like an oncoming lorry you didn't see when you pulled out) and unlocks the full power of the engine.(BMW, Vauxhall, Honda, Toyota)

Adjust the transmission settings on automatic gearboxes, so it shifts up at lower revs.(Volkswagen, BMW)

Activate start/stop mode, which turns the engine off when you're waiting at red lights.(Land Rover, Vauxhall)

Limit or disable some of the more energy-hungry gadgets on board, especially the air conditioning(BMW, Toyota, Honda)

Activate a set of visual cues to encourage more efficient driving behaviour. Think lights that glow red when you accelerate or brake too heavily, and prompts to change gear at lower revs.(Ford, Land Rover)

Worth using?

Possibly. In most cases, the eco settings on cars are designed to automate or encourage a driving style that you can adopt in any vehicle: smooth acceleration and braking, shifting up a gear before the revs get too high, turning off your engine instead of idling and using the aircon sparingly. So if you're already doing this stuff, you might not see much difference. But if you struggle to get in the habit, hitting the eco button takes some of the effort out of it.

Washing machines

On most washing machines the eco mode is either an extra setting you can apply to any of the standard cycles, or a special cycle in its own right.

In both cases, you're altering the way the heating element behaves - either running a longer cycle at a lower temperature, or heating the water to the same temperature but much more slowly. It's not obvious that running something at lower power for longer would save energy, but in this case it seems to work.

The only exception to this is Samsung's 'ecobubble' setting, which froths up the detergent so it can spread through your clothes more easily. Samsung claim that this vastly reduces the amount of heat you need to get things clean, giving you the results of a 40° wash at just 15°. Reviewers seem to be divided on how well it works, but as a new approach to energy saving, it looks pretty promising.

Worth using?

Maybe, but don't expect a huge saving. It's also worth bearing in mind that the eco setting isn't necessarily the most energy efficient cycle you can get from your machine.

Dishwashers

As with washing machines, dishwasher eco programmes save energy by heating the water more slowly over a longer cycle. Some models can also sense how dirty the water is and recycle anything that's clean enough to use again.

This saves heating up more fresh water when it's not needed.

Worth using?

Yes. Which? say that using your dishwasher's eco mode could cut its energy use by up to 20%.

Beware half-load programmes, however - they often use nearly as much energy as a full load. If you can, it's always better to wait until the machine's full before you set it off.

The verdict

Manufacturers could do a much better job of explaining what the eco modes on their products actually do, not least because they're mostly quite good!

But while these settings can be helpful, they're best thought of as one energy-saving tool among many. You'll usually find that the common sense stuff - turning your thermostat down a bit, running full loads in the dishwasher and not driving like an arse - will make a much bigger difference.